Conventionally, an electronic imaging apparatus, for example, a digital still camera has an auto-focus controlling apparatus (referred also to as an auto-focus adjustment apparatus) which converts a subject image into an image signal (image data) by an image pickup device (CCD), and performs a focus adjustment to the subject based on the image signal.
As shown in FIG. 1, in a focus adjustment system by the auto-focus controlling apparatus, a peak value scanning system (so-called climbing AF system or CCDAF system) is generally used, in which by calculating a peak value FVp in a hill-shaped characteristic curve MO acquired by focus evaluation values FV at lens positions fp of a focus lens fL, an optimum focus lens position fbp corresponding to the peak value FVp is acquired.
The focus evaluation value FV is acquired by acquiring high frequency component of the image signal changed by driving the focus lens fL in an optical axis direction at discretely set lens positions fp.
In FIG. 1, arrow A1 indicates a case where the focus lens is driven in a direction from a lens position fp having a small value to a lens position having a large value to acquire the peak value FVp, and arrow A2 indicates a case where the focus lens is driven in a direction from a lens position fp having a large value to a lens position having a small value to acquire the peak value FVp.
This peak value scanning system is a system for searching the optimum focus lens position fbp corresponding to a focusing state where there is lowest defocus by use of the actual image data and therefore the focus lens fL has a high focusing accuracy to the subject.
On the other hand, since it is necessary to search the optimum focus lens position fbp while actually driving the focus lens, this peak value scanning system has a problem in that searching the optimum focus lens position fbp and setting the focus lens fL to the optimum focus lens position fbp take time.
Then, in order to solve the taking time problem, the conventional peak value scanning system has been proposed, in which a ranging sensor (referred also to as an external AF sensor) for directly measuring a distance to the subject is used, the distance to the subject is measured by the ranging sensor and based on a result of the ranging sensor, scanning at a vicinity of a lens position fp corresponding to the ranging result as a target lens position AFfp is performed so that high speed and high accuracy of the focusing control of the focus lens fL can be achieved (see Japanese Patent Application Publication Nos. 2001-141984 and 2001-255451).
The focus lens fL is generally driven by a mechanical driving mechanism (see FIG. 2) including a motor (pulse motor) driven by pulses and a gear.
By the mechanical driving mechanism including the pulse motor and the gear for driving the focus lens fL, the lens position fp of the focus lens fL in the optical axis direction is determined by controlling a pulse number.
However, in the mechanical driving mechanism, since there exists backlash as shown in FIG. 2, even when the pulse number is the same, the lens position when the focus lens fL is driven in the optical axis direction and in a direction of arrow A1 as a normal direction and set to the optimum focus lens position fbp differs from the lens position when the pulse motor is rotated in a reverse direction and the focus lens fL is driven in a direction of arrow A2 and set to the optimum focus lens position fbp.
In the conventional peak value scanning system, in order to detect a peak value FVp of the focus evaluation values FV, while the focus evaluation value FV increases, exceeds the peak value FVp and then decreases, for example, the focus lens is moved in the A1 direction shown in FIG. 3. Accordingly, when the peak value FVp of the focus evaluation values FV is determined by the calculation, the present lens position frp of the focus lens fL exceeds the optimum focus lens position fbp and is positioned at the end lens position fep′ having the focus evaluation value FV smaller than the peak value FVp.
Therefore, it is necessary to return the focus lens fL to the optimum focus lens position fbp judged as a position where a peak of the focus evaluation values exists.
In such a case, in the conventional peak value scanning system, since there exists backlash of the gear, considering the pulse number corresponding to an exceeding amount over the optimum focus lens position fbp and the pulse number corresponding to deviation amount due to backlash, by a pulse number of at least a sum of these pulse numbers or more, the focus lens fL is returned back in the A2 direction and then, by a pulse number required for driving the focus lens fL from a corrected lens position fqp to the optimum focus lens position fbp, the pulse motor is driven to move the focus lens fL in the A1 direction from the corrected lens position fqp to the optimum focus lens position fbp to set the focus lens fL to the optimum focus lens position fbp.
Accordingly, in the conventional peak value scanning system, even if the vicinity of the lens position fp corresponding to the ranging result as the target lens position AFfp is searched, there is a problem in that it takes a time for setting the focus lens fL to the optimum focus lens position fbp.
Furthermore, when changing a rotational direction of the pulse motor, it takes a stop excitation time required for stopping an excitation of the pulse motor.
In the conventional peak value scanning system, in order to set the focus lens fL to the optimum focus lens position fbp, at least twice of the stop excitation time are required.
Therefore, it takes a further time for setting the focus lens fL to the optimum focus lens position fbp.
Furthermore, in a digital still camera having a high magnification zoom lens, even if a perspective range is the same, a focal depth at a telephoto side is small compared to a focal depth at a wide angle side. Accordingly, a scanning range for scanning a peak value by moving the focus lens fL at the telephoto side is larger than that at the wide angle side.
Therefore, every time one focus evaluation value is acquired, the pulse number for driving the focus lens fL toward another lens position fp is increased to reduce the time required for scanning.
Accordingly, if one focus evaluation value FV is acquired after the focus lens fL passes over the optimum focus lens position fbp, and then a decreased value from the peak is detected, an overrun amount of the focus lens fL in each acquired focus evaluation value FV is increased.
As described above, in the conventional peak value scanning system, a control for returning the focus lens by the overrun amount of the focus lens fL after detecting the peak value is required and there is a problem in that a scanning speed in the peak value scanning system is required to be improved.